Bertie Ahern, the Irish prime minister who won three successive elections and played a crucial role in the Northern Ireland peace process, announced his resignation yesterday amid fears that claims about his personal finances could derail the Republic's forthcoming vote on the EU's reform treaty.

Ahern, who will step down on May 6, is expected to be replaced by the deputy prime minister and finance minister, Brian Cowen.

His unexpected announcement, made mid-morning at Government Buildings in Dublin, came after almost 11 years as taoiseach.

In an emotional speech surrounded by cabinet colleagues, Ahern denied he had ever been guilty of political corruption. The body he set up to investigate links between politicians, builders and business figures, the Mahon tribunal, is currently investigating Ahern's personal finances from the early 1990s.

"I have never done anything corrupt in my office," he said. "I know in my heart of hearts that I have done no wrong and wronged no one. I have provided more detail about my private finances than anyone ever in public office."

Senior government sources revealed yesterday that one of the factors behind his decision were fears within his inner circle and the ruling Fianna Fáil party that the vote on the Lisbon treaty, expected in June, would be used as a de facto referendum on Ahern's leadership.

"Fianna Fáil didn't want the referendum on the European treaty to be diverted into an electoral judgment on Bertie Ahern," said a senior Irish government adviser. "Him going now also gives Brian Cowen a chance to sell the yes campaign on its merits for Ireland free from any diversions such as Ahern's personal finances.

"The last thing the Irish government wants is to be blamed for derailing the EU project if the country voted no to get at Bertie Ahern."

He added that Ahern's unexpected departure would also give the new taoiseach time to turn round the government's fortunes before next year's Irish local and European elections.

Cowen, 48, has been a close ally of Ahern. He was told of Ahern's decision to quit on Tuesday night, and last year the prime minister referred to Cowen as his "obvious successor".

Cowen has a reputation for being brusque, but is popular with voters. He is a smoker and Guinness drinker, has admitted to smoking cannabis as a student and enjoys horse racing. He has served as head of most of the important government departments.

Apart from the allegations from the early 1990s concerning his financial affairs, Ahern's 11 years as taoiseach have been dominated by two issues: Ireland's unprecedented economic growth and the peace process in Northern Ireland.

Last night, political leaders on both sides of the Irish Sea paid tribute to Ahern for his role in helping to secure peace and a political settlement between unionists and nationalists in the north.

Tony Blair and Gordon Brown both praised Ahern for his work in bringing about the Good Friday agreement and later the St Andrews' agreement, the latter leading to a once-unthinkable power sharing deal between Sinn Féin and the Democratic Unionists.

"The Northern Ireland peace process would not have happened without Bertie Ahern's contribution," Blair said yesterday.

Among the tributes Ian Paisley, Northern Ireland's outgoing first minister, had some of the warmest words for the 56-year-old taoiseach.

"In sharp contrast with other Irish prime ministers I enjoyed a good working relation with Mr Ahern because he was willing to recognise the position of the unionist population that they had no interest in being part of a united Ireland," he said. "He and I operated as equals, not as one trying to assimilate the other."

John Hume, former leader of the SDLP and Nobel laureate, praised Ahern for the time and energy he devoted to securing peace in Northern Ireland. "His commitment to the greater prosperity of this island has been unstinting both in public and private," Hume said. "He deserves tremendous credit for that."

At a glance

1951 Born in Dublin

1977 Elected to the Dáil

1991-94 Minister for finance

1994 Fianna Fáil leader

1997 Taoiseach

2002 Re-elected

2007 Leads Fianna Fáil to third election victory, begins third term as Irish PM